To provide a sealing structure capable of restricting vibrations of glasses during traveling, and improving a watertight property.

This vehicular sliding window 1 sealing structure comprises an inner partition 2 attached to an inner glass 50 in a state that both glasses 50, 60 are fully opened, an outer partition 3 attached to an outer glass 60, a notch portion 4 formed on a vertical rising piece 14 of a sash 2, and sealing materials 5, 5 fitted in the notch portions 4, 4 and attached to the sash 2. Both partitions 2, 3 are arranged so as to have a clearance 8 between upper and lower ends thereof and upper and lower frames 12, 12 of the sash 2. Extending portions 23, 23 are formed so as to make an vehicular inside and a vehicular outside abut on each other and abut on both glasses 50, 60 when both glasses 50, 60 are fully opened. The sealing material 5 is formed so as to abut on notch side surfaces 25c, 35c and both glasses 50, 60 while both extending portions 22, 23 abut on each other.